Majorly change the way that splitmuxsink collects
incoming data and sends it to the output, so that it
makes all decisions about when / where to split files
on the input side.

Use separate queues for each stream, so they can be
grown individually and kept as small as possible.

This removes raciness I observed where sometimes
some data would end up put in a different output file
over multiple runs with the same input.

Also fixes hangs with input queues getting full
and causing muxing to stall out.